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Calculating wire size requirements for DC Circuits

Remember that we are looking up wire sizes for two reasons:
    (1) To ensure that voltage losses are within acceptable limits, and
    (2) To make sure the wire doesn't get so hot that it melts the insulation (maximum ampacity).


Calculator for unbundled wire in open air, outside of engine spaces.
See below for other environments. Chart is based on USCG, ABYC, and UL standards for marine-rated (105°C dry / 75°C wet) wire.


Ampacity Limits

AWG wire size ampacity limits. However - if your wires are carrying only intermittent current, or carrying current well below the conductor's capabilities, they need not be counted in the bundle. 

CM Area AWG Outside Engine Spaces Inside Engine Spaces
# of Conductors → 1 2-3 4-6 7-24 25+ 1 2-3 4-6 7-24 25+
1,600 18 20 14 12 10 8 17 11.9 10.2 8.5 6.8
2,600 16 25 17.5 15 12.5 10 21 14.7 12.6 10.5 8.4
4,100 14 35 24.5 21 17.5 14 30 21 18 15 12
6,500 12 45 31.5 27 22.5 18 38 26.6 22.8 19 15.2
10,500 10 60 42 36 30 24 51 35.7 30.6 25.5 20.4
16,800 8 80 56 48 40 32 68 47.6 40.8 34 27.2
26,600 6 120 84 72 60 48 102 71.4 61.2 51 40.8
42,000 4 160 112 96 80 64 130 91 78 65 52
66,500 2 210 147 126 105 84 178 124.6 106.8 89 71.2
83,690 1 245 171.5 147 122.5 98 208 145.6 124.8 104 83.2
105,600 1/0 285 199.5 171 142.5 114 242 169.4 145.2 121 96.8
133,100 2/0 330 231 198 165 132 280 196 168 140 112
167,800 3/0 385 269.5 231 192.5 154 327 228.9 196.2 163.5 130.8
211,600 4/0 445 311.5 267 222.5 178 378 264.6 226.8 189 151.2

The chart is based on an amperage capacity multiplier of:
   .70 for 2-3 conductors
   .60 for 4-6 conductors
   .50 for 7-24 conductors
   .40 for 25+ conductors
Plus a multiplier of .85 for engine room heat.


Printable Chart - Voltage Drop at 12 Volts (Double the distances for 24 volt)


12 Volts 3% Voltage drop   Feet                    
  10 15 20 25 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
5 Amps 18 16 14 12 12 10 10 10 8 8 8 6 6 6 6 6 6 6 6
10 Amps 14 12 10 10 10 8 6 6 6 6 4 4 4 4 2 2 2 2 2
15 Amps 12 10 10 8 8 6 6 6 4 4 2 2 2 2 2 1 1 1 1
20 Amps 10 10 8 6 6 6 4 4 2 2 2 2 1 1 1 0 0 0 2/0
25 Amps 10 8 6 6 6 4 4 2 2 2 1 1 0 0 0 2/0 2/0 2/0 3/0
30 Amps 10 8 6 6 4 4 2 2 1 1 0 0 0 2/0 2/0 3/0 3/0 3/0 3/0
40 Amps 8 6 6 4 4 2 2 1 0 0 2/0 2/0 3/0 3/0 3/0 4/0 4/0 4/0 4/0
50 Amps 6 6 4 4 2 2 1 0 2/0 2/0 3/0 3/0 4/0 4/0 4/0        
60 Amps 6 4 4 2 2 1 0 2/0 3/0 3/0 4/0 4/0 4/0            
70 Amps 6 4 2 2 1 0 2/0 3/0 3/0 4/0 4/0                
80 Amps 6 4 2 2 1 0 3/0 3/0 4/0 4/0                  
90 Amps 4 2 2 1 0 2/0 3/0 4/0 4/0                    
100 Amps 4 2 2 1 0 2/0 3/0 4/0                      
120 Amps 2 2 1 0 2/0 3/0 4/0 4/0                      
150 Amps 1 1 0 2/0 3/0 4/0 4/0                        
200 Amps 2/0 2/0 2/0 3/0 4/0 4/0                          
                                       
                                       
12 Volts 10% Voltage drop   Feet                    
  10 15 20 25 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
5 Amps 18 18 18 18 18 16 16 14 14 14 12 12 12 12 12 10 10 10 10
10 Amps 18 18 16 16 14 14 12 12 10 10 10 10 8 8 8 8 8 8 6
15 Amps 18 16 14 14 12 12 10 10 8 8 8 8 8 6 6 6 6 6 6
20 Amps 16 14 14 12 12 10 10 8 8 8 6 6 6 6 6 6 4 4 4
25 Amps 16 14 12 12 10 10 8 8 6 6 6 6 6 4 4 4 4 4 2
30 Amps 14 12 12 10 10 8 8 6 6 6 6 4 4 4 4 2 2 2 2
40 Amps 12 12 10 10 8 8 6 6 6 4 4 4 2 2 2 2 2 2 2
50 Amps 10 10 10 8 8 6 6 4 4 4 2 2 2 2 2 1 1 1 1
60 Amps 10 10 8 8 6 6 4 4 2 2 2 2 2 1 1 1 0 0 0
70 Amps 8 8 8 6 6 6 4 2 2 2 2 1 1 1 0 0 0 2/0 2/0
80 Amps 8 8 8 6 6 4 4 2 2 2 1 1 0 0 0 2/0 2/0 2/0 2/0
90 Amps 6 6 6 6 6 4 2 2 2 1 1 0 0 0 2/0 2/0 2/0 3/0 3/0
100 Amps 6 6 6 6 4 4 2 2 1 1 0 0 0 2/0 2/0 2/0 3/0 3/0 3/0
120 Amps 6 6 6 4 4 4 2 2 1 1 0 2/0 2/0 2/0 2/0 3/0 3/0 4/0 4/0
150 Amps 4 4 4 4 4 2 1 1 1 2/0 2/0 3/0 3/0 3/0 4/0 4/0 4/0    
200 Amps 2 2 2 2 2 1 0 2/0 2/0 3/0 3/0 4/0 4/0 4/0          

All wires should be AWG sizes (not SAE, which are smaller), with insulation rated at 105°C dry, 75°C wet. ABYC standards and USCG certifications should be met. This chart and the adjustments of temperature and bundling are only an approximation of USCG and ABYC standards. For absolute compliance, please see the applicable standards.